1. Field of the Invention
The present invention relates to a polyurethane (PU) foam, and in particular relates to a low-density, fire-resistant polyurethane foam and a fabrication method thereof.
2. Description of the Related Art
Fire-resistant construction materials are classified into three groups: inorganic fiber board, inorganic fiber cotton, and organic fire-retardant foam. Inorganic fiber board is relatively heavy and has a fixed shape which restricts its use and the thermal insulation is unsatisfactory. Inorganic fiber cotton has superior fire resistance and thermal insulation, but has no mechanical strength. Organic fire-retardant foam, such as phenolic foam, is light and has good thermal insulation. However, organic foam generally has poor fire resistance, and tends to shrink and gasify under exposure to flames.
A polyurethane foam incorporated with inorganic fire retardant combines the advantages of organic and inorganic materials, which are, high processability, light-weight, and high thermal insulation for organic materials and excellent fire resistance for an inorganic materials. However, in the conventional art, an inorganic fire retardant is merely physically blended in polyurethane, exhibiting limited improvement in fire resistance. Furthermore, the additional amount of the inorganic fire retardant has restriction. For example, the maximum permissible amount for a 1 μm aluminum hydroxide in polyurethane is only 36.5 wt % (based on the total weight of the fire resistant foam). If the amount exceeds the value, the resulting mixture would not be processable due to unusually high viscosity.
FIG. 1 illustrates a fabrication scheme of a fire-resistant polyurethane foam disclosed by Canadian Patent No. 1222599A1 and U.S. Pat. No. 4,317,889, wherein a fire retardant is first added to a polyol, which is then mixed with a polyisocyanate to cause a foaming reaction. Because the fire retardant is only present in the polyol, the amount of the fire retardant (and therefore the fire resistance) is limited by the volume of the polyol solution.
FIG. 2 illustrates another fabrication scheme of a fire-resistant polyurethane foam disclosed by U.S. Pat. No. 6,010,565 and GB 1472245, wherein a fire retardant, a polyisocyanate, and a polyol are mixed together simultaneously. Because the contact of the polyisocyanate and the polyol initiates the foaming reaction immediately, it is not possible to incorporate a large amount of the fire retardant in such a short period.
FIG. 3 illustrates a further fabrication scheme of a fire-resistant polyurethane foam disclosed by GB 1499168 and EP0308769B1, wherein an open-cell polyurethane foam is impregnated with a cross-linkable latex solution including aluminum hydroxide.
It should be noted that the conventional art uses a single particle size for the inorganic fire retardant. None of the above cited references address the effect of using inorganic fire retardants having different particle sizes.